Cerium

Cerium is one of the least expensive rare earths and is the major component of "mischmetal", used in lighter flints because it catches fire easily when struck. Larger blocks are used for sparking special effects.

Campfire lighter flint.
This is basically a huge, 1/2" diameter x 2.5" long lighter flint, mounted in a case that has a sharp steel blade positioned so you can scrape it against the flint, creating a shower of sparks. The "Spin" and "Rotate" videos are actually video showing what it looks like when you scrape the flint with a file, generating even more sparks.Source: eBayContributor:Theodore GrayAcquired:16 April, 2009Text Updated:16 April, 2009Price: $30Size: 3"Purity: <20%

Advertising set.
This lovely little set of rare earth oxides was made to promote the fact that rare earths really aren't very rare. Once the technology was developed to separate and purify then economically, they became quite common in fact. There is no date on this piece which is a pity, but I would guess it was made in the 1960's.Source:SoCal (Nevada), IncContributor:Theodore GrayAcquired:23 July, 2004Price: $20Size: 8"Purity: >20%

Sample from the Everest Set.
Up until the early 1990's a company in Russia sold a periodic table collection with element samples. At some point their American distributor sold off the remaining stock to a man who is now selling them on eBay. The samples (except gases) weigh about 0.25 grams each, and the whole set comes in a very nice wooden box with a printed periodic table in the lid.

Lump.
This sample arrived with a full set of lanthanides at a time when I was missing europium, terbium, holmium, ytterbium, and of course lutetium.

This very kind donation from Max Whitby of The Red Green & Blue Company in England completed my element collection, to the extent that it gave me a plausible sample of every element one can plausibly have a sample of. (The Red Green & Blue Company is selling a periodic table collection containing similar samples of the same stuff, and if you want a ready-made collection of elements, that's the first place I would look.)

Metal sliver.
This sample of metallic cerium arrived coated in mineral oil in a plastic bag, because it would otherwise likely have oxidized away in transit. I cleaned it up some and put it in a dish covered with fresh mineral oil.
It happened to arrive in my mailbox on the very day Oliver Sacks was visiting the Periodic Table Table, so we got to open it together.Source:Andrew GoodallContributor:Andrew GoodallAcquired:12 November, 2002Price: DonatedSize: 1"Purity: >80%

Optical polishing powder.
Cerium oxide in the form of a very fine powder is used as a polishing compound for optical glass. Supposed it works better than aluminum oxide or other abrasives in certain situations. It's very cheap: A few dollars a pound, which tells you something about how rare the "rare" earths actually are, which is not very.Source:eBay seller fellowsContributor:Theodore GrayAcquired:1 October, 2002Price: $5Size: 1"Purity: <30%Sample Group:Powders

More confiscated Davidite.
This mildly radioactive Davidite ore was confiscated from a student who brought it to school, not realizing that schools tend to freak out about radioactive things, whether they are truly dangerous or not. The original source is United Nuclear and it's perfectly legal.Source:AnonymousContributor:AnonymousAcquired:8 May, 2007Text Updated:9 May, 2007Price: DonatedSize: 1"Composition: (La,Ce,Ca)(Y,U)(Ti,Fe)20O38

Confiscated Davidite.
This mildly radioactive Davidite ore was confiscated from a student who brought it to school, not realizing that schools tend to freak out about radioactive things, whether they are truly dangerous or not. The original source is United Nuclear and it's perfectly legal.Source:AnonymousContributor:AnonymousAcquired:8 May, 2007Text Updated:9 May, 2007Price: DonatedSize: 1"Composition: (La,Ce,Ca)(Y,U)(Ti,Fe)20O38

Ferro-cerium fire starter.
This is a great little gadget, a BlastMatch fire starter from www.survivalinc.com. It consists of a hefty (ca. 1/2" diameter) stick of iron-mischmetall alloy (mischmetall is a mixture of various rare earth elements typically containing a majority of cerium, so this alloy is also known as ferro-cerium). Ferro-cerium is what lighter flints are made of, so this is basically a giant lighter flint, which creates sparks when struck or scraped with something hard. This gadget includes a hard steel blade and a spring mechanism that lets you scrap the blade hard against the ferro-cerium stick just by jabbing the stick down on a hard surface. The net effect is that if you hold it firmly and push it hard down onto a surface, you get a shower of sparks flying off it, enough to light any sort of dry tinder, paper, etc. It's meant to be used as a camp fire starter or survival tool, but if you enjoy showers of sparks, this is way too much fun to leave to wet mornings in the woods.Source:Survival, IncContributor:Theodore GrayAcquired:24 February, 2007Text Updated:24 February, 2007Price: $25Size: 2"Composition:FeCe

Mischmetal.
Rare earths are notoriously hard to separate from one another. In fact the hardest part about discovering some of them was proving that a substance thought to be a single pure element was in fact a mixture of several extremely similar ones.
Since they are chemically so similar, it's not surprising that they also occur together in nature: Ores that contain one inevitably contain several of the others as well. This, combined with the difficulty of separating them, made them quite rare and unusual in pure form, until the development of modern, efficient separation techniques.

But their very chemical similarity also means that in many cases it's really not necessary to separate them in the first place. If cerium will do, then so will lanthanum, or half a dozen others. A case in point is lighter "flints" which are actually made of a mixture of rare earths, primarily cerium and lanthanum, alloyed with iron. The exact ratio of rare earths in a given lighter flint isn't determined by some formula, it's determined by whatever came out of the mine that day. It would be entirely pointless to separate out and use just one in pure form: It would work, by why bother when the raw mixture works just as well?

This mischmetal has not been alloyed with iron, as it would be in a flint: It's a mixture of predominantly cerium (54% cerium is a common for mischmetal), with most of the remainder being lanthanum. Others in the lanthanide series most likely contribute a few percent of the total.

It's very sparky! Just shaving it with a knife produces sparks, a bit like you get when grinding iron on a grinding wheel, except you don't need the grinding wheel. (You can see that I've scraped the oxide coating off one face, a task for which I used a utility knife and file, producing great quantities of sparks in the process.) It's said that blocks like this are sometimes dragged underneath cars to produce a shower of sparks for special effects in movies or performances.

It's a bit like super-sensitive magnesium. Magnesium shavings will also burn, just not spontaneously like those off this block. Blocks of magnesium are commonly sold as camp fire starters, but this stuff would work way better! In fact, to start the shavings off a magnesium fire starter, you use a flint made with this stuff.

When it burns, it burns much the same way as magnesium. Metal fire is very different in appearance from wood, paper, oil, or other types of fire. One reason is that in most fires what's burning is primarily gas driven off from the solid or liquid material. Thus you get bright tongues of flame flickering above whatever is burning. But in a metal fire there is no vapor given off, so only the solid (or if it gets hot enough, liquid) metal is burning. This makes for a very compact point source of light, rather than a spread-out flame. Normal flames also tend to be yellow almost all the time, due in part to the strong yellow-orange emission line of sodium, which is present in some quantity in nearly all natural materials. Metal fires on the other hand tend to be whiter, magnesium being a particularly good example of a very, very white flame.

Another difference is that the metal oxides that build up from burning metal are extremely resistant to heat (magnesium oxide is a common ingredient in high temperature insulation), and totally non-volatile (unlike the carbon dioxide that results from burning organic matter, which is a gas). Thus as metal is burning, it tends to form a crust of oxide around itself which slowly chokes off the flow of oxygen, causing the remaining metal to burn more and more slowly. (I have a story about burning magnesium which includes photographs of this lovely phenomenon.)

I haven't yet set a whole block of this stuff on fire, but I've ordered a couple more and will update when I have pictures of what happens when you actually get a big chunk of it going. I'm betting that blocks of it burn a lot like blocks of magnesium, just faster. Of course I could be wrong: The added rate of reaction could blow off the oxide fast enough to avoid congestion, resulting in a much more dramatic and complete combustion. We shall see.

Monazite Sand.
Monazite is a thorium-bearing mineral that occurs in sand deposits in a number of places around the world. Only a small proportion of the sand in this sample is actually monazite: It is probably somewhat selected compared to normally occurring sand deposits, but not much. It's kind of remarkable, really, that you can collect thorium just by scooping it up with a shovel.Source:Max Whitby of RGBContributor:Max Whitby of RGBAcquired:20 September, 2005Price: DonatedSize: 1.5"Composition: (Ce,La,Nd,Th)PO4